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[libc++] Introduce __forward_as #118168

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Merged
merged 1 commit into from
Dec 14, 2024
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[libc++] Introduce __forward_as #118168

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17 changes: 17 additions & 0 deletions libcxx/include/__utility/forward_like.h
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@

#include <__config>
#include <__type_traits/conditional.h>
#include <__type_traits/is_base_of.h>
#include <__type_traits/is_const.h>
#include <__type_traits/is_reference.h>
#include <__type_traits/remove_reference.h>
Expand Down Expand Up @@ -39,6 +40,22 @@ forward_like(_LIBCPP_LIFETIMEBOUND _Up&& __ux) noexcept -> _ForwardLike<_Tp, _Up
return static_cast<_ForwardLike<_Tp, _Up>>(__ux);
}

// This function is used for `deducing this` cases where you want to make sure the operation is performed on the class
// itself and not on a derived class. For example
// struct S {
// template <class Self>
// void func(Self&& self) {
// // This will always call `do_something` of S instead of any class derived from S.
// std::__forward_as<Self, S>(self).do_something();
// }
// };
template <class _Tp, class _As, class _Up>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _ForwardLike<_Tp, _As>
__forward_as(_LIBCPP_LIFETIMEBOUND _Up&& __val) noexcept {
static_assert(is_base_of_v<_As, remove_reference_t<_Up>>);
return static_cast<_ForwardLike<_Tp, _As>>(__val);
}

#endif // _LIBCPP_STD_VER >= 23

_LIBCPP_END_NAMESPACE_STD
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6 changes: 2 additions & 4 deletions libcxx/include/variant
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Expand Up @@ -1309,14 +1309,12 @@ public:

template <__variant_visit_barrier_tag = __variant_visit_barrier_tag{}, class _Self, class _Visitor>
_LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(this _Self&& __self, _Visitor&& __visitor) {
using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
return std::visit(std::forward<_Visitor>(__visitor), (_VariantT)__self);
return std::visit(std::forward<_Visitor>(__visitor), std::__forward_as<_Self, variant>(__self));
}

template <class _Rp, class _Self, class _Visitor>
_LIBCPP_HIDE_FROM_ABI constexpr _Rp visit(this _Self&& __self, _Visitor&& __visitor) {
using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
return std::visit<_Rp>(std::forward<_Visitor>(__visitor), (_VariantT)__self);
return std::visit<_Rp>(std::forward<_Visitor>(__visitor), std::__forward_as<_Self, variant>(__self));
}
# endif

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